<title>Differential Amplifier</title>
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<applet code=Circuit.class archive=circuit.jar width=640 height=450>
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<param name=pause value=20>
<param name=startCircuit value="amp-diff.txt">
<param name=startLabel   value="Differential Amplifier">
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<p>
This circuit outputs the difference in voltage between two input
signals.  In this case, the first input is a 60 Hz signal, and the
second input is a 60 Hz signal with 120 Hz <a href="http://en.wikipedia.org/wiki/Square_wave">square wave</a> added in.  (In
this simulation, the two signals are added simply by connecting the two
sources in series, which is convenient but not realistic.)
<p>
The second input signal is
driving two resistors, which act as a voltage divider, and the + input
of the op-amp is connected between them, where the voltage is equal to
half the second input signal.  The <a
href="http://en.wikipedia.org/wiki/Operational_amplifier">op-amp</a>
attempts to keep its &ndash; input at the same voltage as the + input.  The two
resistors on top act as a voltage divider, making the &ndash; input
halfway between the first input signal and the op-amp output.
<p>
If V<sub>+</sub> = 1/2 In<sub>2</sub> = V<sub>&ndash;</sub> = 1/2
(In<sub>1</sub> + output), then the output = In<sub>2</sub> -
In<sub>1</sub>, or the difference between the two inputs.

<p>
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